When compatible color television broadcasting standards were generated, advantage was taken of the fact that the frequency space allocated to a black and white video signal is not completely occupied. Instead, because of the interlaced scanning structure of the signal, the brightness information, at baseband, tends to be clustered around multiples of the horizontal line frequency (15,750 Hz for NTSC black and white transmission), and the brightness information in the neighborhood of the multiples of the horizontal line frequency tends to be clustered in multiples of the frame repetition frequency (30 Hz for NTSC black and white transmission). The frequencies between these clusters have less energy. The color information, at baseband, is similarly clustered.
In developing standards for the compatible color television transmission, it was decided to place the clusters of color information between the clusters of brightness information. In the case of the NTSC signal in the United States, the color information was centered between the clusters of brightness information. In other words, the color information is clustered about odd multiples of one half the line rate.
In order to place the clusters of color information between the clusters of brightness information, the color information is modulated onto a subcarrier. The color information forms side bands clustered at frequencies separated by multiples of the line frequency from the subcarrier frequency. The subcarrier frequency was selected to be at an odd multiple (455th multiple for NTSC) of one-half the line rate. Thus, both the subcarrier and the sidebands containing the clusters of color information all are centered between the clusters of brightness information within the band of frequencies allocated to the composite television video signal.
When the composite television video signal is received at the receiver, the color information must be separated from the brightness information to properly display the image being transmitted. To recover luminance and chrominance components, comb filters are utilized to separate the signal components.
The comb filter in the color television receiver separates the signal into bands which should contain color information only and should contain brightness information only. But if these bands do not in fact contain only color information or only brightness information, the filter cannot accurately separate the color information from brightness information.
If the image changes in time, and/or has relatively high detail in the vertical or horizontal direction, the clusters of both brightness and color representative information occupy a larger frequency space than they do in the case of a still, relatively low detail picture. When the brightness signal of an image which is changing in time or has high detail is combined with the modulated color representative signal for such an image, it is possible and probable for a particular frequency to contain both color and luminance information.
In the article by J. O. Drewery "The Filtering of Luminance and Chrominance Signals to Avoid Cross-colour in a PAL Colour System" in BBC Engineering, September 1976, pages 8 to 39, a system is described in which color and brightness information are first combined and then filtered in such a way that the color information occupies a first predetermined frequency space and the brightness information occupies a second predetermined frequency space which does not include the frequency space occupied by the color information. In this way there can be no crosstalk between the color information and the brightness information. A complementary filter in the receiver separates the composite television video signal into a color chrominance component signal which contains no brightness information (i.e. no cross-luminance), and a luminance component which contains no color information (i.e. no cross-color).
The filtering scheme disclosed in this article, however, is a complementary filtering scheme. This means that, in a real system having non-perfect filters, the amount of crosstalk may rise to a -12 dB level at the points where the brightness channel filter response equals the color channel filter response. This is because the filter response characteristics are equal at the -6 dB point, and there are two filters, one at the transmitter and one at the receiver.
With one filter, the characteristics of the channels cannot be tailored to optimize the response of the system. For example, in the NTSC system, the bandwidth of the two color difference signals (I and Q) are different. In the filter circuit disclosed in the article, the response of the two channels must remain equal, even though the bandwidth of the two color difference signals are different.
It is an object of the present invention to provide video signal processing apparatus for combining a brightness representative signal and a color representative signal into a composite video signal in such a manner as to eliminate cross-color and cross-luminance interference.